In a large, heavy pot or a Dutch oven over medium heat, combine the oil and flour. Stirring slowly and constantly, make a dark brown roux, the color of chocolate, 20 to 25 minutes. Add the chopped onions, bell peppers, chopped celery, and 1/2 pound chopped sausage. Season with Creole seasoning. Cook, stirring, until the vegetables are very soft, 8 to 10 minutes. Add the stock and gumbo crabs, stir until the roux mixture and broth are well combined. Bring to a boil, then reduce the heat to medium-low and cook, uncovered, stirring occasionally, 2 hours. Meanwhile, season the remaining seafood with Creole seasoning. Add the seafood and remaining sliced sausage to the gumbo. Cook for an additional 15 minutes. Remove from the heat and let sit for 5 minutes before skimming off the fat that has risen to the surface. Taste for seasonings and adjust. Stir in the green onions and parsley and serve the gumbo in individual soup or gumbo bowls with the rice. Pass the file powder at the table.